Mission 3:
After you kill the 4 guys in the merchandise store ("They seem nice enough") don't follow Passos into the bar. Go right and into the toilets, there's a PK just to the right of the cubicles, past the corpse. This is well out of the way and not recommended for a regular NYM run

Mission 8: At the end, post-Morgue, when you and Passos are behind the podium (?) you both start off in cover. Shuffle to the right, whoop there it is.

Mission 13: There's another PK in the room where you open the security doors (The third 'room' of the level). You start this room taking cover behind a pillar. From here it's in the room at your 3 O'clock position.

Mission 13 again: After you kill the LMG guy you go into a briefing room, then into an Evidence Room, you turn right to take out two guards drawing your attention away from an office on the left-hand-side with a PK on the wall.